For FF continuous, if minimum_im is undefined, set it to 1/4 of the smallest mean value
minimum_im and maximum_im are now mandatory in the DB, so we assume they are always defined.
Merge pull request #200 from gem/set-min-im-if-undefined

For FF continuous, minimum_im and maximum_im are now mandatory in the DB, so we don't need to set their values in case they are undefined